Fibre optic networks often employ tunable technologies both for optical add/drop ports and for transmitters. Transmitters using tunable lasers are desirable for several reasons. First, tunable lasers reduce the number of product variants required to construct the network. For example, a Dense Wavelength Division Multiplexed (DWDM) communications band typically has 80 channels. If fixed wavelength (i.e. non-tunable) lasers are used to drive this channel band, then 80 different variants of the transmitter laser are required. This is problematic, especially for customers who must maintain an inventory of appropriate transmitter components for replacement in the case of failure (sometimes called “sparing”). The second reason is that the use of tunable lasers enables re-tuning the transmission wavelength of any given channel in the system for the purpose of reconfiguration, which in turn enables the implementation of an Optical Add/Drop Multiplexer (OADM).
The Add portion of an OADM can be made tunable by including a tunable filter which is tuned in conjunction with the transmitter laser. Prior to the introduction of practical Digital Signal Processor (DSP) based coherent transmitters, it was commonplace to use Distributed Feedback (DFB) or Distributed Bragg Reflector (DBR) tunable laser designs, which have significant out of band noise in the form of side modes and spontaneous emission. This noise needed to be rejected, which drove the need for filtering the laser output light.
Prior to the introduction of practical DSP based coherent receivers, it was commonplace to use direct detection receivers. In a DWDM system, direct detection receivers require optical filters to separate a desired one wavelength channel from the DWDM signal, and present the separated channel light to the receiver for detection. This type of receiver can detect any wavelength which the optical filter chooses. Therefore, the drop portion of the OADM can be made tunable by including a tunable filter.
However, tunable filters are expensive. Reducing the number of tunable filters is advantageous. With coherent transmitters/receivers, it is possible to reduce or eliminate the filtering from the adds/drops. For example, please refer to PCT/CA2009/001455 titled COHERENT AUGMENTED OPTICAL ADD-DROP MULTIPLEXER and filed on Sep. 11, 2009 which is herein incorporated by reference in its entirety. The result is to replace the optical filters with couplers and splitters which are not wavelength selective.
In a typical OADM, transmitters/receivers and the Add/Drop multiplexer are constructed as separate components, and connected together by manually installed fibre cables. This arrangement allows the user to upgrade a system by adding individual channels over time. However, the manual installation of fibre cables leads to a risk of misconnections due to human error.
The challenge in this case is that in the absence of filtering in the Add/Drop multiplexer, misconnections can be made which place two transmitters of the same wavelength on the same Add/Drop multiplexer. This situation will result in an outage on the affected channel(s).
It is, therefore, desirable to provide a technique for preventing the connection of a transmitter to an optical communications system of a transmitter which is tuned to a channel already in use.